Film resistors are commonly used in printed circuits and include thick film resistors which are conventionally formed by screen printing a resistive material on an insulating substrate and then firing the material and thin film resistors which are conventionally formed by sputtering or vacuum depositing a resistive material on an insulating substrate. In printed circuits it is often necessary to adjust the resistance of the film resistors in the circuit. To increase the resistance of a film resistor the resistor is "trimmed" by forming a cut across the electrical current path in the resistor to make the effective width of the resistor smaller and thereby increase the resistance. The cut may be formed by mechanical abrasion, chemical etching, or laser vaporization of the resistor material. The advantages of laser trimming over mechanical or chemical trimming include very high production rates, greater flexibility in functional trimming and tighter tolerances.
Current methods to laser trim thin film resistors, such as cermet (CrSiO) include:
(1) Pulse trimming--A current is pulsed through the resistor which changes the resistor value by altering the structure and composition of the material. The maximum charge from the original resistor value is approximately 50%. PA1 (2) Laser annealing--The resistor is annealed by a low intensity beam spot which changes the resistor value. The mechanism to change the resistor value is equivalent to pulse trimming since the net effect is to heat the resistor. Again, the maximum change in resistance is approximately 50%. PA1 (3) Network trimming--A resistor pattern is produced with lines connected in both series and parallel loops. The laser deletes connecting loops which alters the resistor length. This method has the capability for making resistors with a large potential resistor range, i.e. several decades, but the resistors require a substantial amount of substrate area. PA1 (4) The resistor material is vaporized in a limited area to decrease the resistor width thereby increasing the resistor value. With conventional methods, change capability is approximately 50% to 100% from the original resistor value.
In the present printed circuit technology, use is made of metallized ceramic modules which are connected on the printed circuit board or card and which contain at least one integrated circuit chip. In one application of interest, it is desired to have a plurality of "so-called" terminating resistors on a one inch square metallized ceramic substrate with the resistors connected between the chip and input/output pins. It is also desired, for example, to have some of the resistors at 750 ohms and some of them at 7500 ohms. In attempting to meet these specifications, thick film paste resistors having a thickness in the order of 240 K angstroms were tried out. In paste resistors, each paste is equal to a certain value and it was necessary to mix more than one paste. Also, when trimming paste the resistance excursion can only be about threefold and still maintain stability. This necessitated the costly stocking of a plurality of substrates having different part numbers. In addition, it was found that the paste resistors had to be applied to one side of the substrate first and then the metallization applied to the other side because the metallization cannot stand up to the firing temperature of the paste, which is in the order of 800.degree. C. This resulted in double masking which was time consuming and made the release of the package too costly.
It became evident that a method was needed which would allow the resistors and metallization to be applied to one side of the substrate and which would allow a substantially larger resistor trimming excursion.